Low, Milky-Way like, Molecular Gas Excitation of Massive Disk Galaxies at z~1.5

TL;DR

This study finds that massive disk galaxies at z~1.5 have low-excitation molecular gas similar to the Milky Way, challenging previous assumptions about high-redshift galaxy gas excitation.

Contribution

First observation of Milky-Way-like low-excitation molecular gas in high-redshift massive galaxies, using CO line observations to characterize their gas properties.

Findings

CO[3-2] flux comparable to CO[2-1], indicating sub-thermal excitation.

Detection of CO[1-0] consistent with higher luminosity than CO[2-1].

Molecular clouds with properties similar to local spirals can explain observations.

Abstract

We present evidence for Milky-Way-like, low-excitation molecular gas reservoirs in near-IR selected massive galaxies at z~1.5, based on IRAM Plateau de Bure Interferometer CO[3-2] and NRAO Very Large Array CO[1-0] line observations for two galaxies that had been previously detected in CO[2-1] emission. The CO[3-2] flux of BzK-21000 at z=1.522 is comparable within the errors to its CO[2-1] flux, implying that the CO[3-2] transition is significantly sub-thermally excited. The combined CO[1-0] observations of the two sources result in a detection at the 3 sigma level that is consistent with a higher CO[1-0] luminosity than that of CO[2-1]. Contrary to what is observed in submillimeter galaxies and QSOs, in which the CO transitions are thermally excited up to J>=3, these galaxies have low-excitation molecular gas, similar to that in the Milky Way and local spirals. This is the first time that such conditions have been observed at high redshift. A Large Velocity Gradient analysis suggests that molecular clouds with density and kinetic temperature comparable to local spirals can reproduce our observations. The similarity in the CO excitation properties suggests that a high, Milky-Way-like, CO to H_2 conversion factor could be appropriate for these systems. If such low-excitation properties are representative of ordinary galaxies at high redshift, centimeter telescopes such as the Expanded Very Large Array and the longest wavelength Atacama Large Millimeter Array bands will be the best tools for studying the molecular gas content in these systems through the observations of CO emission lines.